Magnetic amplifiers



May 24, 1960 R. E. MORGAN MAGNETIC AMPLIFIERS 6 Sheets-Sheet 1 OriginalFiled Feb. 5. 1954 D. C, SIG/VH1.

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$568 fltt'or'ne y May 24, 1960 R. E. MORGAN MAGNETIC AMPLIFIERS 6Sheets-Sheet 6 Original Filed Feb. 5. 1954 THERNOL'JUPL! Morgan x. 5 .0s m m m a? In #7615 .flatorwey CS Patent 7 2,938,159 MAGNETIC AMPLIFIERS.Raymond E..Morgan, Schenectady, N .Y., assignor to Gen- :eral Electric,"Company, a corporation of New York Continuation of Lapplication Ser.No. 408,347, .Feb. 5, .1954. This application Jan. 31, 1958, Ser. No.712,587

12 Claims. (Cl. 323-89) This applicationfis a continuation of mycopending application Serial Number 408,347,, filed February '5, T954,and assigned to the present assignee.

My invention relates to magnetic amplifiers. More particularly itrelates to magnetic amplifiers which are characterized by highstability, gain, and speed of response.

' 'Theneed for precise'measurement and control in such fields asprecision calorimetry, temperature measurements, optical pycnometry,spectroscopy, physiology, geophysics, meteorology, oceanography, and thelike has created a demand for highly stable magnetic amplifiers whichare capable of amplifying extremely low level signals,'and, at the sametime, have desirable gain and speed of response. Conventionalnon-self-saturating magnetic amplifiers do not have enough gain or ahigh enough speed of response for such exacting work. Whileselfsaturating magnetic amplifiers or 'amplistats, as they are sometimescalled, have a useful gain and speed of response, they are not stableenough for use at very low signal strengths. "Typical zero drift for aself-saturating magnetic amplifier is of the order of 10 or 10- wattsdue to ambient changes, such as that of temperature. The drift inself-saturating magnetic amplifiers is due largely to the use ofrectifiers in the amplifier circuit, and also to core characteristics.

An object of my invention is to provide magnetic amplifiers having goodstability and desirable speed of response.

A further object of my invention is to provide mag netic amplifierswhich arecapable of accurately detecting very low signals.

Another object of my'invention is to provide magnetic amplifiers havingfavorable stability and response characteristics which eliminate the useof rectifiers.

A still further object of my invention is to provide magnetic amplifierswhich may be cascaded without the use of rectifiers.

Another object of my invention is to provide magneticamplifiers havingdesirably low drift.

Other objects will become apparent and the invention betterunderstood'from a considerationof the following description and thedrawing in which:

Fig. 1 shows a simplified magnetic amplifier of my invention having aD.C. signal;

Fig. 2 shows .the hysteresis loop'for the magnetic amplitier of Fig. 1;

Figs. 3a and 3b show the pulse power supply for the magnetic amplifierof Fig. l in reference to the power .bus sine wave;

Fig. 4 shows the characteristic of a single stage D.C. signal magneticamplifier of my invention without bias, such as that shown in Fig. 1;

Fig. 5 shows the circuit arrangement of a magnetic amplifier 'of' myinvention with D.C. signal and AC. bias; V

' Figs. 6a, 6b, "and 60 show the flux positions'in the '2 main core.of'the magnetic amplifier of Fig. 5 at various times of the powersupply cycle;

Fig. 7 shows the characteristic of a single stage D.C. signal magneticamplifier of my invention with an AC. bias;

Fig. 8 shows one of my improved magnetic amplifiers with a pulse signal;

Fig. 9 shows the hysteresis loop for the magnetic amplifier of Fig. 8;

Figs. 10a, 10b, and show the bias and pulse power supplies timecorrelation for the magnetic amplifier of Fig. 8;

Fig. 11 shows another embodiment of my invention;

Figs. 12a, 12b and 12c show the time flux conditions for a pulse signalpulse relaxation magnetic amplifier;

'Fig' 13 shows a four-stage magnetic amplifier; and

Fig. 14 shows the overall gain characteristics for a four-stage magneticamplifier of my invention.

Briefly stated, my invention, in its broadest sense, relates to magneticamplifiers in which a power pulse drives the core into saturation.Following this there is a relaxation period during which themagnetization core returns to a level determined by the control signalor bias or both. The output power is determined by the pulse remainingafter saturation. Operation equivalent to the push-pull operation ofconventional self-saturating magnetic amplifiers or amplistats isachieved using a single magnetic core by comparing the alternatepositive and negative pulses. Control is achieved by controlling thepoint along the steep portion of the hysteresis loop from which eachcycle starts. Such magnetic amplifiers may aptly be referred to as pulserelaxation magnetic amplifiers.

Referring now 'to Fig. 1, there is shown an embodiment of my inventionin which the magnetic amplifier is operated by a D.C. signal from aconvenient source not shown. Among its advantageous features is its corewhich is much smaller in cross-sectional area than that of the ordinaryself-saturating magnetic amplifier. It is particularly useful inapplications having a high impedance input and, unlike the half-waveself-saturating amplifier, has no rectifier. In the embodiment of Fig.'1 magnetic core 1 has wound on one leg a power or output winding 2which is connected through impedance load '3 to pulse power supply 4,which latter may be of types well-known to those skilled in the art.This pulse power supply .is energized by a power bus not shown. Woundaround the other leg of core 1 is control or signal winding 5 connectedto a D.C. signal source. Impedance 6 in series with the D.C. signalwinding prevents the signal circuit from loading "the power winding bytransformer coupling action.

The magnetic amplifier of Fig. 1 is useful in installations where thereis a low level high impedance D.C. signal source which does not requirea bias such as is found in ion chambers and some tachometer equipment orwhere the signal range is limited.

In order better to understand the operation of the magnetic amplifier ofFig. 1, reference is made to Fig. 2 and Figs. 3a and 3b. Fig. .2represents the hysteresis loop of the core 1. Since there is no changein core dimensions, the axes for the hysteresis loop are expressedsimply as flux versus ampere turns. For ease of explanation, the loop,as are all other hysteresis loops in'the drawings, is shown infullyidealized form as comprising four straight lines. In .Fig. 3a isshown one full cycle of a typical power bus voltage supply which may beof any frequency, for example, up to 1000 cycles per second or higher.Shown in Fig 3b are the pulses derived by the pulse power supply 4 fromthe power bus. The pulses in Fig. 3b are shown as vertical straightlines,

to make the change possible.

since their narrow width in terms of time, which is only a few, forexample, two degrees of the power bus cycle, makes it impractical todraw them to scale in the figure. In other words, the ratio of pulseduration to repetition period is very small (much less than one-half).The duration of these pulses is thus several times less than the periodbetween pulses. The letters a, b, 0, etc. in Fig. 2 and Figs. 3a and 3brefer to relative points in time as between the hysteresis loop and thepower bus sinusoid and the power pulse power supply.

For purpose of explanation, let us. start with point b on the hysteresisloop of Fig. 2. This starting position is controlled by the D.C. controlor signal ampere turns and has been selected for purposes ofillustration as a point three-fourths of the way towards positivesatura' tion from negative saturation. As the pulse supply voltagerises, the flux rises from point b towards saturation. The reactor isdesigned so that the pulse supply can swing the flux over the entirerange of the hysteresis loop and no more. Hence, the time required toraise the flux from point b to saturation is one-quarter of the time ofduration of the pulse. Accordingly, the core remains in saturation forthree-quarters of the pulse length and full current can flow as long asthe core remains in this state of saturation. After the supply pulseends, as indicated at point d, the core magnetization drops to that ofthe signal ampere turns. Since no voltage is applied from point d topoint e the core remains at the same position on the hysteresis loopuntil the negative pulse is applied. This negative pulse, occurring atpoint e, is similar to the positive pulse except that it is reverse inpolarity. Points 2, f, and g of the negative pulse correspond to pointsb, c, and d, respectively, of the positive pulse.

As the pulse supply proceeds in the negative direction, the flux isdriven down to point f. The core is not driven into but merely up tonegative saturation, since the volt-time integral of the supply pulse isby design just sufiicient to affect the flux excursion from positive tonegative saturation. The transition from point 1'' to point g is madesubstantially instantaneously at the end of the negative pulse, no timedelay being involved since there is no flux change. Since the D.C.signal is demanding a higher number of ampere turns than occur at pointg,

the core returns to point b as at the start. However, the point g topoint b passage does require a flux change, and the small current thusinduced in the power circuit tends to impede the change. Hence, adefinite period of time hereinafter known as the relaxation period isrequired The duration of this relaxation period is, of course, severaltimes greater than that of the supply pulse. The time constant for therelaxation period is equal to the total load circuit inductance dividedby the load circuit resistance and consists of the unsaturatedinductance of the power winding plus the inductance of the load (or nextstage control) winding divided by the load circuit resistance. Duringthis period the pulse supply inductance and resistance are very small.In practice these load circuit constants are determined in theparticular design so that the flux can settle or relax to the pointdetermined by the signal point b before the positive pulse reoccurs.

A decrease in D.C. signal ampere turns drops point b to a lower value offlux. During the positive pulse there is more flux to change betweenpoint b and positive saturation, thus requiring a longer time to reachsaturation and leaving a smaller portion of the pulse time during whichload current can flow. An increase in D.C. signal ampere turns resultsin the reverse effect or less time for the pulse to reach saturation,producing more load current. In both instances the negative power pulseremains the same, it being absorbed in swinging or changing the corefrom positive to negative saturation. When the D.C. signal is reversedso that the starting point b 4 is on the descending side of the looprather than on the ascending side, the roles of the positive andnegative pulses are interchanged.

A plot of average output current, averaged during the supply pulse,versus signal current, provides a characteristic similar to that of aself-saturating magnetic amplifier connected in push-pull. Thischaracteristic is shown in Fig. 4, either half of the characteristicbeing like that of a simple self-saturating magnetic amplifier.Furthermore, the ampere turns required to swing the reactor over itsfull characteristic in the positive control region is exactly the sameas would be required for the simple selfsaturating magnetic amplifier oramplistat with perfect rectifiers, and the same is true for thecharacteristic in the negative control region. Thus Fig. 4 is abidirectional characteristic with a dead spot or low gain section in thecenter which results from the supply pulses, both positive and negative,being fully absorbed by the reactor as long as the control ampere turnmagnitude is too small to reach either the ascending or descending sidesof the hysteresis loop. The low gain section in the center of thecharacteristic shown in Fig. 4 is undesirable for many uses and iscorrected by a cross bias winding similar to that used inself-saturating magnetic amplifiers arranged in push-pull. With thecross bias, both positive and negative output pulses are produced andthe net output is the diifereuce between these two pulses. A rectifierdiscriminator may be used to convert the output to D.C. Suchdiscriminators are well known to those skilled in the art. Since themagnetic amplifier of the present invention achieves its push-pullcharacteristic with a single core, the common technique of employing aD.C. cross bias may not be employed. However, an A.C. cross bias can beused, since the pulse relaxation amplifier output is the differencebetween positive and negative pulses. The A.C. bias is a pulse biasapplied between the time points a and b and applies a positive volt-timeintegral to the core just preceding the positive signal pulse and anegative volt-time integral just preceding the negative signal pulse. Asine wave may be used to produce this effect. A magnetic amplifier ofthe pulse relaxation type employing an A.C. bias is shown in Fig. 5.

Shown in Fig. 5 is a pulse relaxation amplifier having A.C. cross bias.Wound on core 7, which may be either three-legged or separated, are A.C.cross bias windings 8 and 9, which are connected to an A.C. bias source,D.C. control windings 10 and 11 and output or power winding 12. Winding11 is optional and may be used as a con.- trol, reference, or biaswinding as desired. Windings 10 and 11 are connected to power sources asshown. A power circuit series filter 13 may be placed in circuit withoutput coil 12 and pulse power supply 4. Filter 13 is optional and, ifused, comprises a capacitor 14 shunted by a resistor 15. The extra righthand core in the amplifier of Fig. 5 performs the same function as thesimple series inductance often used in the control winding circuits ofconventional self-saturating magnetic amplifiers and not in a mannerwhereby its characteristics are compared with those of the main core. Itwill be noted that A.C. cross bias windings are provided on both cores.They are connected as shown to prevent flux cancellation in the D.C.control winding which prevents the existence of induced voltage andhence circulating currents in the control circuit. This provisionminimizes the bias requirements and prevents any disturbance of thesignal source. Similarly, the RC filter 13 improves the A.C. biasrequirements by reducing induced power circulating currents. In effect,filter 13 is a high pass filter presenting a low impedance to the powerpulse or short pulse and a high impedance to currents induced by theA.C. bias which is a longer pulse. As pointed out above, filter 13 aidsthe operation but is not necessary.

two-legged core may .be tusedainstead of the threerlegged core .shown:in Fig. :5 with the :signal and .biaswindings on one legithereof. "The:operationof the magnetic amplifier of Fig. 5 may be better understoodby reference to Figs-6a, 6b, and 6c. .Fig. 6a illustrates the-operationof the.A.C. bias in terms of :the hysteresis loop. The AC. bias isadjusted so that .atpoints b and e, the flux is set half way betweenpositive and .negative saturation. Then .as the supply pulse is appliedthe flux rises'from point b and by the time point e is reached the coreis well into positive saturation as' shown in Fig. 2. has :much as halfof the flux excursion is already then accomplished by the bias pulse,only half of the supply pulse is needed to drive the core on to positivesaturation. Hence, the load current flows during the second half of thepulse, producing half the average output pos- 'sible for the full pulseduration. The same sequence of events occurs for the negative half cyclefor points e, f,- and :g. Thus the outputs from the positive andnegative pulses are equal, and each is at half of its-maximum averagevalue to give a net "output ofzero.

In Fig. 6b a positive D.C. signal is applied in addition "to the A.C.bias. As a result the D.C. signal shifts points b and e toward positivesaturation. For purposes of illustration these points have been selectedsuch that one-fourth of the supply pulse then is required to drive thecore into positive saturation and load current flows during theremaining three-fourths of the pulse. During the negative half cycle theefiect has the reversed proportions, three-fourths of the pulse timebeing required to reach negative saturation leaving only one-fourth ofthe pulse for the load current flow. Output during the positive pulse isthus increased to 75 percent of its maximum value, and during thenegative pulse is decreased to percent of its maximum value. The netoutput is positive.

Shown in Fig. 6c is the effect of the negative D.C. signal where the netoutput is negative. Here the effect is just the opposite of thatexperienced with the positive D.C. signal.

The complete push-pull characteristic with bias applied is shown in Fig.7.

The magnetic amplifier of Fig. 5 is useful in applications where a lowimpedance low level D.C. signal source is present such as in athermocouple or low impedance shunt and a bias such as in a conventionalordinary selfsaturating magnetic amplifier is indicated.

The foregoing embodiments-of my invention have been devoted to magneticamplifiers of the pulse relaxation type having a D.C. signal. However,such amplifiers may also be made having a pulse signal of relativelyshort duration several times less than that of the'period between outputcircuit power supply pulses. Since pulse signals induce voltages in allreactor windings, additional consideration must be given to the magneticcoupling which occurs between windings. Therefore, the bias in powerwinding circuits must be designed for high impedance or voltagecancellation during the signal pulse. 'Insuch casethe signal source isnot loaded by the winding and amplification is unimpaired.

A simple pulse relaxation magnetic amplifier using a unidirectionalsignal is shown in Fig. 8. On one leg of core 17 is signal winding 19energized by a pulse signal source as shown and having in seriestherewith impedance 20 which prevents the loading of the power windingby transformer coupling action.

On the other leg of core 17 is wound output or power winding 18.Connected in series with impedance load 21 and pulse power supply 24 isimpedance 22. Impedance 22 serves to prevent signal loading by the powercircuit. Shunted across impedance 22 is switch 23 which is closed.duringthe power pulse so that no supply voltage is lost during thisperiod in impedance 22. It will be noted that this simple amplifier hasno bias. The pulse supply24-1ags-the signal or pulsezfrom the pulsesignal source :into 19, and both of the pulse signals may operate .fromthe same power bus which is not shown.

The switch-23is open between points B and D and closed betweenpointsE-and G of Fig. 9.

The operation of the pulse relaxation amplifier of Fig. .8 may best beunderstood by reference to .Fig. 9 and 10a, 10b, and 100. Fig.9 showsthe ideal hysteresis loop of the amplifier core. Figs. 10a, 10b, and 10cshow respectively the power bus sinusoid, the signal pulse power supply,and the load pulse power supply. The lettered points in the figures arerelative in all cases. At point B of Fig. 9, the flux is at negativesaturation and zero ampere turns, this being the residual condition leftby the preceding negative power pulse. The signal pulse applied at thispoint has sufiicient amplitude and duration to make transition to pointC which, for purposes of illustration, is selected as three-fourths ofthe flux excursion from negative to positive saturation. The signalpulse is removed at point D. Thereafter the flux remains unchanged andthe coremoves immediately to its residual condition at point E. Novoltages or current are applied to any winding on the core betweenpoints D and B .so the core experiences no magnetic change during thisperiod. At point E the power pulse is applied, driving the core intopositive saturation, one fourth of thepower pulse being required tofinish driving the flux to saturation, leaving three-fourths of thepower pulse duration for load current flow, which is 75 percent maximumaverage output per power pulse. After the power pulse has ceasedat'point G the core drops back to the residual condition of zero ampereturns at positive saturation and remains in this condition until point Lis'reached. This is true .since the negative pulse illustrated in Figure10b is not supplied. The power pulse at point.M drives the flux tonegative saturation and the residual holds the flux in this conditionuntil point B of the next cycle.

A decrease in the signal pulse magnitude causes a .lower flux level atpoint C, thus requiring more of the positive power pulse to drive thecore to saturation, and leaving a lesser amount of time for outputcurrent flow, which means a decrease in output. The reverse occurs foran increase in signals. If the signal 'is reversed in polarity andshifted in phase to point J, the negative power pulse reacts in the samemanner as described above for the positive pulse. Plotting of the loadcurrent averaged over the power supply pulse only versus the averagevalue of the signal pulse results in a characteristic similarto thatshown in Fig. 4. As in the simple pulse relaxation amplifier with D.C.signal, it is desirable to have maximum or high gain at zero signal.Again, an AC. bias is used to eliminate the low gain region to produce apush-pull characteristic like that of Fig. 6. This arrangement is shownin Fig. 11.

In Fig. 11, three-legged core 25 has wound on one leg thereof mainoutput or power winding 26, and on another leg another identical powerwinding 27 connected as shown. On the center'leg of the core is pulsesignal winding 28 which is connected to a pulse signal source. Pulse orAC. bias winding 29 is also connected to a pulse bias power sourcethrough resistor 29a. Winding 30, known as a wipe-out winding from itsfunction later to be discussed, is also wound around one leg of the core32 and connected through resistor 30a to a power source. The leg 31 ofthe core 25 having the'main power winding 26 is known as the main core,while that having the other power winding 27 is known as the auxiliarycore. In series with the main power winding 26 is resistor 33 which actsto prevent loading of the power winding by transformer coupling action.Resistors 29a and 30a have a similar role in the bias and wipe out"circuits respectively. In circuit between the other power winding andits pulse power supply 34 is capacitor 35 which acts, as a filterpresenting a high impedance to slow. changing voltages with lowfrequency components 7 such as bias function, or wipe-out function. Theuse of this filter is optional. It will be noted that the circuit shownin Fig. 11 is identical to the D.C. signal circuit of Fig. except thatwindings 27 and 30 are added.

In the case of the circuit of Fig. 11, the bias pulse is of sufficientmagnitude and phase to drive the core into saturation before the signalpulse is applied. Furthermore, the signal consists of both positive andnegative pulses as shown in Fig. 10b. It is important that the magnitudeof'these two outputs be properly chosen in order to establish theportion of the transfer characteristic on which the second or pulsedstage operates. The high gain part of the characteristic is desired. Asin the case of the self-saturating magnetic amplifier the bias can beapplied positive and the signal negative and vice versa. Variations inthe output caused by changes in saturation in the flux level of the coreare eliminated by using the bias positive and the signal negative, thatis the signal is applied in opposition to the power pulse. The bias isapplied ahead of the signal pulse and is removed the instant the signalpulse ends. The phase relation is fixed, since all elements of powersupply output are derived from the power bus cycle. The operation of thecircuit of Fig. 11 may be better understood by reference to Figs. 12a,12b and 12c, wherein Fig. 12a shows the hysteresis loop of the maincore, Fig. 12b shows the pulse signal and bias in reference to the sinewave of the power supply, and 120 shows the hysteresis loop of theauxiliary core with the MMF drawn in reverse fashion. As the sine waveof the power bus rises through point A in Figs. 12a, 12b and 12c, andalso Fig. 10a, the bias is applied and remains until point B, thusdriving the core into positive saturation, as at B in Fig. 10a. Thesignal pulse is applied in the opposite direction of the bias, drivingthe flux from point B to point C, which moves the flux three-fourths ofthe way to negative saturation. At point D the signal pulse and bias areremoved simultaneously. Minor variations or minor differences in thetime removal result in very slight differences in flux. Since few or noampere turns are applied to the core between the end of the power pulseat D and the occurrence of the power pulse at B, there is no change inthe residual flux level until the power pulse is applied. The powerpulse drives the core into positive saturation between points E and G.Since threefourths of the power pulse is required to complete the fluxchange to reach positive saturation, one-fourth of the pulse time isleft for the load current flow, or twentyfive percent of the maximumoutput. Operation for the next half cycle of the power bus is merely theinverse of that just described starting from positive saturation andproducing a negative pulse of output current.

The switch 23 and impedance 22 shown in Fig. 8 are obviated in Fig. 11by the incorporation into the auxiliary core 32 of an equivalentimpedance and using this core saturation to provide the switchingaction. During the time the signal is applied, the voltage induced inthe power winding 26 of the main core completely cancels that induced inthe auxiliary core; thus, in effect, performing the function ofimpedance 22 of Fig. 8. If the auxiliary core 32 were left unchangedfrom point B to E it would also completely cancel any effects producedby the signal output. This cancellation is, however, prevented by thewipe-out pulse provided by winding 30.

Referring to Fig. 12c, the hysteresis loop of the auxiliary core 32 isdrawn with the magnetomotiveforce, that is ampere turns, scale reversedso that a simultaneously comparison between it and the hysteresis loopof the main core, Fig. 12a, can readily be made. The bias drives theauxiliary core into negative saturation between points A and B, whilethe main core is driven into positive saturation. As the signal isapplied, the same flux changes result in both cores, but in oppositedirection. After the signal pulse there is no flux change in the maincore between points D and E, but the auxil- 8 iary core is driven intopositive saturation by the wipeout pulse. When the positive pulse of thebias power supply occurs at point F, it finds the auxiliary core alreadyin saturation.

It should be noted that the auxiliary core is saturated without changingthe flux of the main core. The pulse bias applied to the auxiliary coreis applied at such a low rate, that is long time and small amplitude,that the induced voltage in the auxiliary windings are extremely small.Hence, they have little effect on the main core flux through thewindings and circuits coupled to the auxiliary core circuits. The smallcirculating currents are normally too small to move the core to the edgeof the hysteresis loop and cause a flux change. The high pass capacitorfilter 35 provides an impedance sufiicient to keep the circulatingcurrent small enough to prevent a flux change in the main core. The mainpower pulse passes through the capacitor, furthermore, with negligibleloss, and this main pulse has a higher rate of rise and, hence, containshigher frequency components than the bias pulse. However, the use ofcapacitor 35 is optional.

As is well-known, conventional non-self-saturating magnetic amplifiersrequire output rectification in order to be successfully cascaded.However, as pointed out heretofore, the pulse relaxation magneticamplifier of my invention has such gain and speed of response that thestages of cascaded amplifiers may be coupled by A.C. pulses rather thanD.C. signals. Thus, any number of stages may be cascaded without the useof rectifiers. Shown in Fig. 13 is a four stage pulse relaxationmagnetic amplifier which is used to record on meter 38 variations intemperature sensed by thermocouple 36. It will be appreciated that anysignal source may be used. The first input stage of the cascadedamplifier has a D.C. signal and is similar to the amplifier shown inFig. 5 except that winding 11 of the latter is omitted. The second,third, and fourth stages are simply signal pulse relaxation magneticamplifiers of the type shown in Fig. 11, the output of the fourth stagebeing in circuit as shown with a rectifier discriminator which convertsthe A.C. signal received into a D.C. output for operating meter 38. Suchrectifier discriminators are well-known to those skilled in the art. TheRC filter 39 comprising resistor 40 and capacitor 41 functions in amanner similar to that described above for filter 13, such filters beingoptional. Capacitors 35 are also optionally used as filters, whileimpedances 42 and 43 are used to prevent loading of induced voltages inthe bias windings. Pulse power supplies 44 and 45 provide power pulsesfor their respective stages while bias supplies 46 and 47 providenecessary bias.

While only four stages are utilized in the embodiment of Fig. 13, itwill be appreciated that any number of stages may be used to achieve adesired output level having very low drift. Rectifiers are notintroduced until the signals are of such strength that rectifier driftis not injurious. It will also be appreciated that the D.C. output maybe used to drive other devices, such as a motor, through, for example, aconventional self-saturating magnetic amplifier stage which raises thepower to the necessary level. The first stage of Fig. 13 which issimilar to Fig. 5 may also be used above as the first stage of anelectronic amplifier, the load being the grid resistance of theelectronic amplifier.

It will be seen that the cascading of stages of pulse relaxationmagnetic amplifiers is somewhat similar to that of conventionalpush-pull self-saturating magnetic amplifiers in that for a given changein signal or a given change in output of the first stage, the secondstage or succeed: ing stage can be made to increase or decrease itsoutput, thus reversing signal sense between stages. In Fig'. 13 thecircuit is cascaded so that the preceding stage always reverses thesense of the following stage.

The pulse relaxation amplifiers of my invention. are

=veryrlow .Fig. 114 showsthe overall gaincharacteristic for-1a :typicalfour-stage amplifier in terms of average volt output versus :averagevolt-input. Inasmuch 'EIS theinput :andtoutput'resistance levels aredifierent, the overall amplifier voltage gain has less significance than.an overallpower gain. .If this is. computed from the linear portion ofthe gain characteristic using the .re- .sistance of input'and outputcircuits, the powergainmay well be of the .order of two :million in atypical case. The linear portion of the amplifier ;will accommodate a:maximum .input signal of .110" watts, while .delivering an output inexcess of 10 watts. It will, at once, be apparent that this comparesvery favorably with conventional self-saturating magnetic amplifiers.

Speed of response in the pulse relaxation amplifier is faster than thatof the conventional self-saturating magnetic mnplifier or amplistat. Thecircuit of Fig-13 has -a maximum delay of one cycle for the first stageplus vone-ifourth cycle for the three .following stages for a total ofone and three-fourths cycles. The time constant J's-measured in cyclesat any power supply frequency. T o thisis added the time constant :ofthe first stage con- .trol winding. Similarly, the minimum delaypossible is :One and one-quarter cycle, plus control winding delay.Obviously, the amplifier time constant exclusive of the input winding isinversely proportional to the power bus frequency. A faster speed ofresponse can be obtained .by power supply design as well as byincreasing the power bus frequency. For example, an amplifier whichoperates from '60 cycles and a pulse power supply producingfour pulsesper second may be altered .to-obtain equivalent one thousand cycleoperation by operating the power supply from ,60 cycles and designing itso as to derive 66 pulses-per cycle. Methods of accomplishing this willbe obvious to those skilled in the art. The role of the power supplythen is expanded to that of a frequency multiplieras well as a :pulseproducer. The maximum cycle delay of the first stage can also be reducedby special design involving additional cores.

The drift of the pulse relaxation amplifiers of my invention is verylow. Tests were made to estimate the influence of temperature variationsand powersupply variations on the output and gain of the pulserelaxation amplifierof this invention. The drift level was determined byobserving the output of-theamplifier with the input signal held at zero.An input signal, which, when amplified, gave the same magnitude ofoutput as that given by the drift level, defined the minimum signallevel of the amplifier. Power supply voltage was varied plus or minus 30percent, and temperature of the first stage was cycled about -70 C. and+140 C. Maximum drift during this period was watts with the gaincharacteristic showing complete continuity. This compares with typicalzero drift for a self-saturating magnetic amplifier of the order of 10''or 10- watts.

The noise level of the magnetic amplifiers is over 60 decibels belowthat of the conventional self-saturating magnetic amplifier. This isbrought about largely by eliminating rectifiers and using the same corecoil and circuits for both polarities of signal in push-pull operation.

It will be obvious to those skilled in the art that the pulse relaxationmagnetic amplifier has many other uses than those specifically describedabove. Instead of having a control or signal winding as such woundaround the core of such a magnetic amplifier, any source of magneticflux or any material which disturbs the flux leakage from the core maybe used to transmit signals to the amplifier. For example, the magneticamplifier of my invention may be used as a thickness gauge by measuringthe influence on the amplifier of pieces of metal, whose thickness is tobe measured, which will disturb the flux leakage path. The proximity tothe core of such material or of flux producing material may also bemeas- 10 m'ed. My'magnetic amplifiermay:alsobe.used to indicate compassdirections without the use of mechanically moving :parts .as in .a gyrocompass, for example. Illustrativeof this use in its simplest form, thecontrol winding of Fig. 71 may be eliminated and the magnetic field of:the earth permitted to act as a signal source. With the coil heldverticallyand pointedin any direction, say

north, the earths magnetic field .will produce an output of onedirection, and when the coil is pointed in the. opposite direction theoutput will also be in the opposite direction. Headings .in betweennorth and south will produce proportional efiEects. Suitable means maybe used to indicate the heading, .or, for example, drive an autopilot.

While I have described above certain specific embodimerits of myinvention, other embodiments and uses therefor will occur to-thoseskilled in the art. It is my intention, therefore, to protect by thefollowing claims all embodiments of my invention which do not departfrom the spirit and scope thereof.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A magnetic amplifier comprising a saturable mag netic core having anoutput winding connected in series with a load, control means forcontrolling the level of flux in said core, means for supplying periodicvoltage pulses of one polarity across said output winding to producefiux in said core in one direction and for introducing periodicpulses offlux in said core in a direction 0pposite to said one direction, saidlatter flux pulses each occurring during respective periods intermediatesaid voltage pulses to relax said core to a flux level controllablybysaid control means, said pulses being of relatively short durationcompared to the interval between pulses to allow sufiicient time betweensuccessive pulses for said control means to set the flux in said core toa predetermined control flux level.

2. The magnetic amplifier of claim 1 wherein the duration of the periodsbetween said voltage pulses is several times greater than that of saidvoltage pulses.

3. The magnetic amplifier of claim 1 wherein the amplitude of said fluxpulses .is substantially equal to that required to drive the corebetween its positive and negative saturation conditions.

4. A magnetic amplifier comprising a saturable magnetic core having anoutput winding connected in series with a load, control means forcontrolling the amount of flux in said core, a load circuit includingmeans for passing periodic pulses of output current of one polaritythrough said output winding and for supplying periodic voltage pulses ofopposite polarity to said output winding during periods intermediatesaid current pulses to relax said core to a flux level controllable bysaid control means, each of said pulses being of relatively shortduration compared to the interval between pulses to allow sufficienttime between successive pulses for said control means to set the flux insaid core to a predetermined control flux level.

5. The magnetic amplifier of claim 4 wherein the duration of the fluxrelaxation periods between said voltage and current pulses is severaltimes greater than that of said voltage pulses.

6. A magnetic amplifier comprising a satu-rable magnetic core having anoutput winding in series with a load, a control winding, a symmetricallyconducting output circuit including a first pulse power means in serieswith said output winding for supplying across said output windingperiodic voltage pulses of alternating polarity each having a durationseveral times less than the period between said voltage pulses, and acontrol circuit including a second pulse power means in circuit withsaid control winding for supplying across said control winding signalpulses occurring during the periods between said voltage pulses whichprecede the voltage pulses of one polarity, said signal pulses eachhaving a duration sub- '11 stantially less than that of the periodbetween said voltage pulses.

7. A magnetic amplifier comprising a magnetic core, output winding meanson said core, pulse power means in circuit with said output means toform a power circuit and to saturate said core by means of periodicvoltage pulses of alternating polarity each having a duration severaltimes less than the period between said voltage pulses, control means onsaid core, means to energize said control means, biasing means on saidcore, and means to energize said biasing means with periodicallyreversing biasing potentials to provide a net output through said powercircuit during both positive and negative polarity voltage pulses.

8. A magnetic amplifier comprising a magnetic core. output winding meanson said core, pulse power means in circuit with said output means tosaturate said core by power pulses of alternating polarity each having aduration several times less than the period between said power pulses,the portion of said output winding circuit external said output windingbeing symmetrically conductive to alternating polarity currents, controlwinding means on said core, means to energize said control windingmeans, bias winding means on said core, and means to energize said biaswinding means, the control and bias winding means being arranged andconnected to cancel induced voltages in one another.

9. A magnetic amplifier comprising a magnetic core, a plurality ofoutput windings in series relation on said core, said output windingshaving in circuit therewith a first pulse power supply adapted toprovide a voltage pulse followed by a relaxation period, control meanson said core in circuit with a second pulse power supply, biasing meanson said core incircuit with a third pulse power supply, the pulse powersupplies being so arranged that the power pulses from said first pulsepower supply lag those from said second pulse power supply, the pulsesfrom said third pulse power supply being in advance of the pulse fromsaid second pulse supply and relieved when the latter pulse ceases.

10. A magnetic amplifier comprising a main magnetic core and anauxiliary magnetic core, first output means on said main core and secondoutput means on said auxiliary core, a first pulse power supply adaptedto provide a voltage pulse followed by a relaxation period to energizesaid first and second output means, said first and second output meansbeing so connected that voltages 12 induced in said first output meanscancel those induced in said second output means, control means linkingsaid main and auxiliary cores in circuit with a second puls'e powersupply and bias winding means linking said main and auxiliary cores incircuit with a third pulse power supply.

11. A magnetic amplifier comprising a magnetic cor comprising a mainmagnetic core and an auxiliary magnetic core, first output means on saidmain core and second output means on said auxiliary core, a first pulsepower supply to energize said first and second output means and adaptedto provide a voltage pulse followed -by a relaxation period, said firstand second output means being so connected that voltages induced in saidfirst output means cancel those induced in said second output means,control means in circuit with a second pulse power supply linking saidmain and auxiliary cores and bias means in circuit with a third pulsepower supply linking said main and auxiliary cores, the pulse powersupplies being so arranged that the power pulse from said first pulsesupply lags those from the second pulse power supply, the pulses fromsaid third pulse power supply being in advance of pulses from saidsecond pulse power supply, and winding means on said auxiliary core incircuit with a fourth pulse power supply whereby said winding means isenergized after the control pulse from said second pulse power supplyceases.

- 12. A pulse relaxation magnetic amplifier comprising: a saturablemagnetic core; a load winding on said core; a symmetrically conductiveload circuit 1 for connection across a load including means connected inseries with said load winding for supplying periodic voltage pulses ofalternating polarity across said load winding, said pulses each having aduration several times less than the period between pulses and having avolt-time integral sufficient to drive said core between positive andnegative saturation conditions; and means for controlling the amount ofpower delivered to said load by said power pulses after saturation ofsaid core comprising control winding means on said core for controllingthe level of flux in said core during the interval between pulses.

References Cited in the file of this patent UNITED STATES PATENTS

